Natural gas is currently produced at sites remote from the utilization sites and is commonly liquefied so that it can be carried over long distances by tanker, or stored in a liquid form.
The methods used and disclosed in the prior art, particularly in patents U.S. Pat. No. 3,735,600 and U.S. Pat. No. 3,433,026, describe liquefaction methods principally comprising a first stage in which the natural gas is precooled by vaporizing a coolant mixture, and a second stage that enables the final natural gas liquefaction operation to be conducted and the liquefied gas to be obtained in a form in which it can be transported or stored, cooling during this second stage also being provided by vaporization of a coolant mixture.
In such methods, the fluid mixture used as the coolant fluid in the external cooling cycle is vaporized, compressed, cooled by exchanging heat with an ambient medium such as water or condensed air, expanded, and recycled.
The coolant mixture used in the second stage in which the second cooling step is performed, is cooled by heat exchange with the ambient coolant medium, water or air, then the first stage in which the first cooling step is performed.
After the first stage, the coolant mixture is in the form of a two-phase fluid having a vapor phase and a liquid phase. Said phases are separated, in a separating vessel for example, and sent to a spiral tube heat exchanger for example in which the vapor fraction is condensed while the natural gas is liquefied under pressure, cooling being provided by vaporization of the liquid fraction of the coolant mixture. The liquid fraction obtained by condensation of the vapor fraction is subcooled, expanded, and vaporized for final liquefaction of the natural gas, which is subcooled before being expanded by a valve or turbine to produced the desired liquefied natural gas (LNG).
The presence of a vapor phase requires a condensation operation for the coolant mixture in the second stage, which requires a relatively complex and expensive device.
The proposal has also been made in Patent FR-2,734,140 by the applicant of operating under selected pressure and temperature conditions to obtain, at the output of the first coolant stage, a fully condensed single-phase coolant mixture.
This brings about constraints which can be burdensome for process economics, particularly because the pressure at which the coolant mixture used in the second stage is compressed can be relatively high.